This post is dedicated to Mel.
The close relationship between humans and dogs is something of an evolutionary mystery. Is the dog a slave, or a parasite? Have we adapted them to our purposes, or have they exploited us for free food and shelter? Either way, we are so close it can hurt.
Slavery
Enslavement is fairly common amongst ants. Slave-making ants raid the nests of another species and carry off their young. The larvae mature, become attached to the new nest, and do the work for their "kidnappers."
Humans likewise take pups from their mothers at a few weeks old and allow them to grow up in the home as though they were humans. This is possible because all dogs are highly social to begin with. Moreover, we have artificially selected dogs that serve our purposes by breeding animals that are not aggressive, that are highly sociable, and that remain rather playful throughout their lives.
There is a very different reason for two species to live close together, which is that the apparent slave is really a parasite.
Parasitism
The cuckoo is a well-known bird example of a brood parasite who smuggles an egg into a host's nest and lets the host do all the hard work of rearing it. For this to work, the gape pattern of the young cuckoo has to match the brightly colored throat markings of the host species that stimulate feeding.
Biologists suspect that the 50,000 year association between humans and dogs began when their ancestors, wolves, began scrounging meat scraps from around the campfire where meat was cooked. Humans could then domesticate them and use them to provide defense against enemies, or to alert on prey animals.
At this time humans had a sophisticated tool kit, including bows and arrows and spear throwers that allowed them to kill from a distance. They were such efficient killers that our ancestors slaughtered most of their large prey to the verge of extinction everywhere they roamed - a phenomenon known as the Pleistocene overkill. That was bad for large prey animals but it meant lots of scraps for opportunistic canines.
Dogs are astute scroungers and have an impressive capacity to catch food falling from a table before it reaches the ground.
Evidence of cross-species dependence
One of the curious features of slavery in ants is that the slave-maker species loses the ability to do its own work. Since humans began using dogs to detect prey at a distance, our own sensory capacity has also declined as indicated by a measurable reduction in relevant sensory areas of the brain.
This would suggest that humans have enslaved dogs rather than being parasitised by them. Conversely, there is no evidence of any such decline in capacity for dogs who have no trouble going back to independent living as illustrated by the success of dingoes in Australia.
Perhaps the most interesting aspect of human dependence on dogs is how much we rely upon them as companions.
The best family member?
A large proportion of dog owners accept that the pet is a family member (about 40 percent). This is not surprising given that dogs often share our living space, and even sleep in our beds. This practice is very ancient because various human groups used dogs for warmth on cold nights according to anthropologists
Dogs can be trained to be very useful in many ways as illustrated by companion dogs for the blind and physically disabled. The presence of dogs is now recognized as therapeutic in nursing homes and hospitals.
From that perspective, we can acknowledge that the dog is not just a family member but, in some ways, an ideal family member.
The dog provides unconditional love and is apparently incapable of with holding affection. It is also incapable of telling lies. By those criteria, it is not just a family member but, possibly, the most satisfactory family member.
For these and other reasons, the dog is often the center of family interactions and a couple may communicate by interacting with the pet much as they might communicate through a child.
Why a dog's death is so devastating
Whatever the evolutionary reasons, humans and dogs are as emotionally close as it is possible for different species to be. That is why the loss of a dog can be so devastating. On the one hand, we have merely lost a pet that is easily replaced. On the other hand, we have lost not just a family member but perhaps the only one who never deceived us, never withheld affection, and never criticized us.